Peak-to-average power ratio (PAPR) issues are problems associated with high power peaks occurring in signals to be processed. They are historical problems in wireless system design and critically affect broadband communication. PAPR may be represented by the Equation (1), wherein max|s(t)|2 represents a highest peak of a signal, E|s(t)|2 represents an average power of the signal and L represents a number of subcarriers.
                              P          ⁢                                          ⁢          A          ⁢                                          ⁢          P          ⁢                                          ⁢          R                =                                            max              ⁢                                                                                      s                    ⁡                                          (                      t                      )                                                                                        2                                                    E              ⁢                                                                                      s                    ⁡                                          (                      t                      )                                                                                        2                                              ≈                      O            ⁡                          (              L              )                                                          (        1        )            
Orthogonal frequency division multiplexing (OFDM) is a multi-carrier signal generation method whereby data are sent simultaneously over N equally spaced carrier frequencies using Fourier transform techniques for modulation and demodulation. By proper choice of frequencies, OFDM can squeeze multiple modulated carriers into a prescribed band while preserving orthogonality to eliminate inter-carrier interference (ICI) or inter-symbol interference (ISI). This approach has been proposed or adopted for many wireless applications including digital audio broadcasting, digital terrestrial television broadcasting wireless LANs and high-speed cellular data. Techniques for implementing OFDM are well known.
However, a significant disadvantage of employing OFDM for wireless applications is the potentially large peak-to-average power ratio (PAPR) characteristic of a multicarrier signal with a large number of subchannels.
For example, a baseband OFDM signal with N subchannels has a PAPR of N2/N=N, for N=256, PAPR=24 dB. When passed through a nonlinear device, such as a transmit power amplifier, the signal may suffer significant spectral spreading, interchannel interference, in-band distortion, orthogonal loss and increased bit error rate (BER). With the increased interest in OFDM for wireless applications, reducing the PAPR is necessary for implementing OFDM.
For wireless applications, efficient power amplification is required to provide adequate area coverage and to minimize battery consumption. A conventional solution to the PAPR problem in OFDM systems is to use a linear amplifier. However, the linear amplifier is inefficient and is operated with large backoff from its peak power limit to reduce the distortion introduced by clipping, further reducing efficiency. Also, amplifier backoff attenuates the transmitted signals, thereby placing greater demands on receiver sensitivity.
Thus, what is needed is a method and apparatus for reducing PAPR in an OFDM signal which reduces the PAPR substantially without a significant decrease in system efficiency or an increase in system cost.